Please beware! Existing Motion 2 files with any level of reasonable complexity work EXTREMELY SLOW or not at all in Motion3. A discussion with an Apple tech concluded that Motion3 handles movement and particle generation very differently and does not work well with old files.

Apple recommendation: render existing files in Motion2 for use as .mov clips in new FCP projects and rebuild composites from scratch in Motion3 for sequences that will need changes and frequent re-rendering.

NOT A GOOD THING!